# Partition an array based on a condition

Split items into [matching, not matching] in one pass.

Canonical: https://1loc.dev/array/partition-an-array-based-on-a-condition/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Usage lines executed in Node.js 22 and headless Chromium; no external services
Updated: 2026-10-28

## Implementation

```javascript
function partition(values, predicate) { return values.reduce((groups, value) => (groups[predicate(value) ? 0 : 1].push(value), groups), [[], []]); }
```

## Usage

```javascript
partition([1, 2, 3, 4, 5], n => n % 2); // → [[1, 3, 5], [2, 4]]
const events = [{ to: 'ana@example.com', status: 'delivered' }, { to: 'rui@example.com', status: 'bounced' }];
const [delivered, other] = partition(events, event => event.status === 'delivered');
other; // → [{ to: 'rui@example.com', status: 'bounced' }]
```

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

## Limits

- The predicate’s result is tested for truthiness: n % 2 puts odd numbers first.
- Object.groupBy (ES2024) is the built-in for more than two groups.

The function does not validate its arguments. Add checks where it receives untrusted input.

## Source and license

Original: https://github.com/phuocng/1loc/blob/14b481cf60dff1620ab1bb99a43b1f68068cdd9b/contents/partition-an-array-based-on-a-condition.mdx
Source file (this edition): https://github.com/akamind/1loc/blob/0a77f131b22d2ba3b7ec1728846f900c22821678/contents/partition-an-array-based-on-a-condition.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: TangSY
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Documented as a reference with executed usage lines. Original MDX retained as inert source evidence.